2. WHAT SHAPES THE SEDs OF GALAXIES?

There are two fundamental factors to be considered here:

the intrinsic emission spectrum produced by all stellar populations in
a galaxy

the propagation of photons through the ISM.

The intrinsic spectrum depends on the star formation history and
the metallicity evolution of a galaxy. We will not expand on this here since
this is covered by the review talk of Leitherer
[34].
The propagation of photons through the ISM depend on:

the amount of dust and its distribution with respect to the stars

the optical properties of grains

the amount of neutral gas and its distribution with respect to the
young stars

The neutral gas affects the line emission component of SEDs; this topic is
covered in detail in the talk of Kewley et al.
[27].
The optical
properties of grains, which mean their absorption and scattering properties
throughout the UV-submm range, depend on the composition and size
distribution of the grains. This topic is covered in detail by the review
talk of Dwek [17]
(see also the talks by Li
[33]
and by Gordon
[23]),
and is only briefly touched upon here, in
Sect. 4. The amount of dust and its
distribution with respect to the stars has received little attention
until recently, even though the distribution of dust is the single most
important factor affecting the propagation of photons in galaxies. To
illustrate this statement one only has to recall that a fixed amount of
dust distributed on a scale of a few parsec around stars will have the
same effect as 1000 times more dust distributed on kpc scales in the
disk. Therefore this review will mainly focus on the effect of the
relative geometries of stars and
dust on the SEDs. Previous reviews related to this topic include: Calzetti
[9],
[10],
Popescu & Tuffs
[44],
Kylafis & Misiriotis
[30].

We will start with very simple
geometries and increase the complexity until we have identify a minimum
degree of complexity that can account for observed broad-band
UV/optical/FIR/submm SEDs. We will also consider the effect of these models
on the UV/optical/FIR/submm surface brightness distributions. In terms
of the dust emission, we will place most emphasis on the FIR emission,
rather than, for example, on the MIR emission. This is firstly because
most of the energy
absorbed by grains is re-radiated in the FIR, and secondly, because the FIR
colours of a galaxy depend on the strength of the radiation fields in a
galaxy, and therefore more directly constrain the propagation of photons
in the disk.

Our entry point is to consider geometries with cylindrical symmetry,
which are
essential for the description of disk galaxies. Spherical symmetry is a more
reasonable approximation for the description of dwarf galaxies
(e.g. Galliano et al.
[22])
and starburst galaxies (e.g. Witt et
al. [56],
Gordon et al.
[24]).
The simplest model is the infinite slab/sandwich. A sandwich model, not
incorporating scattering, was used by Disney et al.
[13]
to investigate the attenuation-inclination
behaviour of spiral galaxies. This work first emphasised the strong
effect of
the relative scaleheights of stars and dust on the attenuation. Another
version of the sandwich model, this time including scattering, was used
to calculate the energy balance between the emission and re-emission of
light in the pioneering work of Xu & Buat
[61].
In the Xu & Buat
formulation there is only one free parameter, the face-on optical depth,
which is adequate to account for the energy balance. Apart from its
application to the integrated emission from galaxies (Buat & Xu
[4],
Xu et al.
[63]),
this particular model was used by Xu & Helou
[62]
in the modelling of the large-scale dust
heating and cooling in the diffuse medium of M 31.
A common drawback, though, of models involving infinite slab/sandwich
geometries is that they cannot predict the shape of the observed FIR/submm
SEDs. Understanding the FIR colours is not only a matter of academic
concern,
but also provides a further dimension to the predictive power of models,
since the FIR colours directly probe the strength of the radiation fields,
and, as we shall see, strongly depend on physical quantities of interest,
such as SFRs.

In order to also fit the FIR colours, one needs to consider more realistic
geometries, where by realistic we mean incorporation of
finite disks, bulges and small scale structures:

finite disks: stars and/or dust

bulges: stars only

small scale structures: stars and/or dust

Finite disks are usually described by double exponentials in both radial and
vertical direction.
Bulges can be described by a variety of forms: de Vaucouleurs,
truncated Hubble, spherical with King profile/exponential.
We note here that the exact choice
of bulge geometry has little effect on the shape of the globally integrated
SED, provided that the bulk of the luminosity of the bulge is emitted
within an area much smaller than the disk.
Small scale structures have been described as small scale dust
clouds/clumps (depending on terminology)
which, according to the model, may or may not be physically associated with
young stars. As we shall see, this different treatment has strong
consequences on the prediction of the FIR colours.
All or combinations of these geometrical components have been employed by
the models introduced in the following papers: Kylafis & Bahcall
[29],
Bianchi et al.
[8],
Bianchi et al.
[6],
Xilouris et al.
[58],
Silva et al.
[50],
Granato et al.
[25],
Kuchinski et al.
[28],
Popescu et al.
[46],
Matthews & Wood
[35].
All these models were used to account for the optical SEDs, but only
three of them (Bianchi et al.
[6],
Silva et al.
[50],
Popescu et al.
[46])
were used as the basis for a self-consistent
calculation of both the stellar and dust emission SEDs. Recently,
self-consistent calculations of attenuation and re-emission by dust
grains in galaxies have also been done in the work reported by Baes et al.
[3]
in this volume, and have started to be incorporated in population synthesis
models, such as those of Piovan et al.
[43]
and Rocca-Volmerange
[48],
also as reported in this volume.